Ampacities and Mechanical Properties of Rectangular Copper Busbars

Introduction

"Busbar systems" refers to conductors that take the form of a bar or bars of copper conductor. The bars may be exposed or enclosed. The system may have one or more joints to assure proper length and configuration and one or more take-off points connected to end-use equipment.

The tables that follow provide ampacity of copper busbar conductors for sizes typically found in the USA under the temperature-rise conditions specified in the table, along with physical and mechanical properties. Also included are DC ampacity tables provided by ATIS (Alliance for Telecommunications Industry Solutions) for sizes and configurations typically found in the telecommunications industry.

For energy-efficiency considerations, the design of the busbar system should be based on a 30C rise above ambient, or less. Temperature rises above 65C are not recommended and are not energy-efficient.

A more complete guide to busbars and design considerations can be found in the U.K. Copper Development Associations Publication No. 22, Copper for Busbars.

The ampacity tables presented here are for rectangular busbars of Copper No. 110 whose surface condition is similar to that of busses at typical installations. Ampacities were calculated using a nominal conductivity of 99% IACS and should also be applicable to other coppers with the same electrical conductivity. Listed for 60 Hz at temperature rises of 30, 50, and 65°C above ambient, they were determined from accurate emissivity coefficients measured by calorimetric techniques. The methods are described in " Electrical Coils and Conductors", by H.B. Dwight (McGraw-Hill Publishing Co., New York, 1945, Chapter 19).

Ampacity Calculations - Accurate emissivity is essential because it is used to calculate the heat dissipated by radiation, a factor in the general equation for ampacity:
equation where I is ampacity (amp), WR is heat dissipated by radiation (watts), WC is heat dissipated by natural convection (watts), and R is resistance (ohms) at operating temperature and 60 Hz.

Table 1. Ampacities of Copper No. 110 Busbars - Ampacities in this table are for busbars having an emissivity of 0.4.

This was observed on samples exposed for 60 days in an industrial environment, and it is probably identical to that of busbars in service.

Direct-current ampacities may differ from AC ampacities because of AC skin effect: equation where I DC is DC ampacity (amp), I AC is AC ampacity at 60 Hz (amp), and S is the skin effect ratio at 60 Hz.

Table 2. Mechanical Properties of Copper No. 110 Busbars - This table lists properties useful in calculating such characteristics as stiffness and deflection that are often required by designers of busbar systems.

Table 3. Quick Busbar Selector - Knowing the ampacity, designers and estimators can get the approximate busbar size. Ampacity of the busbar selected must then be verified by checking table 1.

Table 4. Effect of Emissivity and Number of Busses on Ampacity - Data here show how higher emissivities improve ampacity. Multiple busses also affect ampacity in a nonlinear relationship. Ampacity may be raised by increasing heat dissipation through the use of convection cooling or surface treatments. Surface treatments which improve emissivity are available.

DC Ampacity Tables - These tables list the DC ampacity of copper busbars in the sizes and configurations most often found in the telecommunications industry. They have been provided by the Alliance for Telecommunications Industry Solutions ( ATIS) for the convenience of the reader.

US Sources of Busbar

Typical Busbar Sizes